1. Field of the Invention
The present invention relates to a method for calculating the output characteristic of an optical tip array which forms an image (latent image) on a recording medium with a light shutter array made of PLZT or an LED array and to an image forming apparatus.
2. Description of Related Art
Conventionally, various kinds of optical write heads which turn on and off light for each pixel with use of a light shutter array made of PLZT or an LED array have been used to form images (electrostatic latent images) on a silver-salt print sheet or film or an electrophotographic photosensitive member. For formation of images without unevenness, such an optical write head needs to be subjected to measurement of the quantity of light outputted from each element and to correction in quantity of light according to the measurement result.
Meanwhile, for formation of a multi-tone image, it is effective to modulate the pulse width of a driving signal which is supplied to the elements. However, since the elements vary in output characteristic, it is difficult to carry out an accurate light-quantity correction to each of the elements. The variation in output characteristic among the elements seems to be caused by distortion and errors in the measurements of PLZT tips which occur in the manufacturing processes. Because of the variation in output characteristic, for example, when 512 elements are supplied with an identical driving voltage, the quantities of light outputted from the elements vary in an extent of xc2x115% to xc2x120%. Further, because of the variation in shape among the elements, the elements vary in half-wave voltage (a voltage to make an element emit a maximum quantity of light). Accordingly, the output characteristic of each element while the pulse width applied thereto is modulated for reproduction of a plurality of tone levels is peculiar to the element itself.
Conventionally, while the elements are driven under a specified condition (a condition to reproduce one tone), the quantity of light outputted from each of the elements is measured, and correction data are produced based on the results. However, the correction data do not reflect the actual output characteristics of the elements, and accurate corrections cannot be made.
Moreover, since the spectral sensitivity characteristic of the photosensor for light-quantity measurement such as a photomultiplier is different from that of the recording medium such as a print sheet or film, light-quantity correction data produced based on the results of the measurement do not agree with the spectral sensitivity characteristic of the recording medium, thereby resulting in failure in formation of a quality image.
An object of the present invention is to provide an image forming apparatus which can reproduce a multi-tone image on a recording medium accurately.
Another object of the present invention is to provide a method for calculating the output characteristic of each element of an optical write device accurately based on results of light-quantity measurement carried out at only several tone levels.
Further, another object of the present invention is to provide an optical write device which can match correction data produced based on the results of light-quantity measurement carried out by a sensor to exposure of a recording medium for formation of a quality color image even if the spectral sensitivity characteristic of the sensor is different from that of the recording medium.
In order to attain the objects above, according to the present invention, a method for calculating the output characteristic of each element of an optical write device which drives a plurality of optical elements arranged in a main scanning direction individually in accordance with multi-tone image data comprises the steps of: measuring the quantities of light outputted from each of the optical elements while driving the optical elements for reproduction of at least three different tone levels; and calculating an approximate expression which indicates the output characteristic of each of the optical elements from the measured values.
According to the present invention, the output characteristic of each of the optical elements is calculated from the results of light-quantity measurement which is carried out at the light-quantity levels for three to five tone levels. The approximate expression calculated in the method indicates the output characteristic with an extremely small error, and accurate light-quantity correction data can be produced from the expression. Consequently, a quality image without unevenness can be formed. In this method, because the characteristic of each element in reproducing different tone levels itself is simulated, accurate correction data can be produced from the calculated expression even if the driving voltage for the measurement varies. Further, since during the light-quantity measurement, the optical elements are driven for reproduction of only several tone levels, it is not necessary to provide a large-scale expensive circuit for the measurement, and driver ICs for two-value image formation which are modified for the measurement can be used.
According to the present invention, the number of tone levels at which light-quantity measurement is carried out depends on the form of the approximate expression to be calculated. For example, if a quadratic expression is to be calculated, the quantities of light outputted from each element which is driven for reproduction of at least three tone levels are measured, and if a cubic expression is to be calculated, the quantities of light outputted from each element which is driven for reproduction of at least four tone levels are measured. By carrying out light-quantity measurement at tone levels near points of inflection of the actual output characteristic of each of the optical elements, an expression which indicates the output characteristic more accurately can be calculated. Further, prior to the light-quantity measurement, a voltage which makes many of the optical elements output their respective maximum quantities of light at a duty of 50% is found out, and the voltage is applied to the optical elements during the light-quantity measurement. Based on the results of the measurement, correction data with small errors with respect to all the optical elements can be produced.
Further, the present invention relates to an optical write device which drives a plurality of optical elements arranged in a main scanning direction individually in accordance with data about three primary colors to write an image on a recording medium, and the device according to the present invention comprises: light-quantity measuring means which has a photosensor for measuring the quantity of light outputted from each of the optical elements; and adjusting means which makes up the difference in spectral sensitivity characteristic between the photosensor and the recording medium.
In the structure, the spectral sensitivity of the photosensor and that of the recording medium are made clear beforehand, and the difference between the photosensor and the recording medium in spectral sensitivity characteristic is made up at the time of light-quantity measurement and/or at the time of exposure of the recording medium by the adjusting means. The adjusting means is a color switching filter and/or a color correction filter provided in a light source section, means for switching the intensity of light emitted from the light source (for example, voltage switching means) or means for switching the sensitivity of the photosensor (for example, means for switching the accelerating voltage applied to a photomultiplier used as the photosensor, means for switching the amplifying constant for the output signal of the photosensor).
Thus, since the difference between the recording medium and the photosensor in spectral sensitivity characteristic is made up, correction data which are produced based on light-quantity measurement by use of the photosensor matches exposure of the recording medium, thereby resulting in formation of a quality color image.
An image forming apparatus according to the present invention comprises: at least one light emitter; a driver which is connected to said light emitter so as to drive said light emitter in accordance with driving image data; a light-quantity detector which detects quantities of light emitted from the light emitter, the light-quantity detector detecting a plurality of quantities of light emitted from the light emitter which is driven for reproduction of different tone levels; and a controller comprising a first control section which calculates an approximate output characteristic of the light emitter based on the quantities of light detected by the light-quantity detector and which stores the approximate output characteristic in a memory, and a second control section which inputs image data and generates the driving image data based on the inputted image data and the approximate output characteristic stored in the memory.